Grinding machine with a re-truing device

ABSTRACT

A grinding machine wherein a CBN grinding wheel is rotatably carried to grind a workpiece on a work support device and is provided with a re-truing device, in addition to a truing and dressing device rotatably carrying a truing roll and a counter roll. When a truing and dressing instruction is generated, the CBN wheel is first trued with the truing roll, is then dressed by free abrasive grain supplied between itself and the counter roll and is finally re-trued with a diamond impregnated truing bar carried on the re-truing device. A re-truing infeed of the truing bar against the CBN wheel is considerably less than a truing infeed of the CBN wheel against the truing roll, and therefore the re-truing is effected on cutting edges of the wheel abrasive grain damaged by the free abrasive grain in dressing, whereby the cutting edges are made somewhat dull.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a grinding machine with devices fortruing and dressing a grinding wheel made of hard material, such ascubic boron nitride (CBN).

2. Description of the Prior Art

For the purpose of truing and dressing such CBN grinding wheels, therehas recently been developed a truing and dressing device which, aftertruing a CBN wheel with a truing roll, having diamond particles securedto its circumferential surface for reforming the wheel surface, dressesthe trued CBN wheel by supplying free abrasive grain between the sameand a counter roll so as to cut away bond material and to therebyprotrude the wheel abrasive grain from the remaining bond material ofthe wheel surface. Using such a truing and dressing device, surfaceroughness of the grinding surface of the CBN wheel trued and dressed bythe device relatively quickly deteriorates in proportion to the increasein the number of workpieces being ground, as indicated by the dottedline (A) of FIG. 5, and this raises one of the problems of the priorart, namely that the wheel life, or the truing and dressing interval ofthe CBN wheel, is short.

In order to solve such a problem, the present inventors made variousexperiments, from which it was found that the free abrasive grain indressing, when cutting away the bond material of the wheel surface tocause the wheel abrasive grain to protrude somewhat, acted also on thewheel abrasive grain which had been trued, to damage or lift the samefrom the remaining bond material. From the experiments, it was furtherfound that, in cases where the CBN wheel was re-trued after truing anddressing, a considerably long life could be expected, as indicated bythe solid line (B) of FIG. 5.

SUMMARY OF THE INVENTION

Accordingly, it is a primary object of the present invention to providean improved grinding machine capable of utilizing a CBN grinding wheelover an elongted wheel life thereof, as well as finishing workpieceswith highly enhanced accuracy of surface roughness.

Another object of the present invention is to provide an improvedgrinding machine with a device for curing or correcting cutting edges ofwheel abrasive grain which is damaged or lifted from the remaining bondmaterial of a wheel surface by free abrasive grain in a previousdressing.

Another object of the present invention is to provide an improvedgrinding machine of the character set forth above wherein, in additionto truing and dressing, re-truing is also effected on a CBN grindingwheel so as to eliminate faults created on the wheel surface in aprevious dressing.

Briefly, according to the present invention, there is provided agrinding machine which comprises, in combination, a bed, a wheel headslidably mounted upon the bed and adapted to rotatably carry a grindingwheel made of cubic boron nitride, work support means for rotatablysupporting a workpiece, feed means for feeding the wheel head toward andaway from the work support means, truing means for truing said grindingwheel, dressing means for dressing the wheel in cooperation with freeabrasive grain, re-truing means for retruing the wheel, and controlmeans for operating the truing means, the dressing means and there-truing means in order when receiving a truing and dressinginstruction.

In operation, the truing means reforms a grinding surface of the wheel,and the dressing means removes bond material from the surface, incooperation with the free abrasive grain, so as to thereby protrudewheel abrasive grain from the remaining bond material of the surface.Following dressing, the retruing means is operated to true cutting edgesof the wheel abrasive grain, so that faults created on the cutting edgesby the free abrasive grain in the previous dressing can be eliminated.

BRIEF DESCRIPTION OF THE DRAWINGS

Various other objects, features and attendant advantages of the presentinvention will be readily appreciated as the same becomes betterunderstood from the following detailed description of a preferredembodiment, when considered in connection with the accompanyingdrawings, in which:

FIG. 1 is a fragmental plan view of a grinding machine according to thepresent invention;

FIG. 2 is a fragmental sectional view of the apparatus, taken along theline II--II of FIG. 1;

FIG. 3 is a hydraulic circuit diagram of an apparatus for controllingtruing, dressing and re-truing operations;

FIG. 4 is an electric circuit diagram of the apparatus for controllingthe operations;

FIG. 5 is a graphical representation illustrative of a relationshipbetween surface roughness of a grinding wheel surface and the number ofground workpieces; and

FIGS. 6A to 6E are explanatory views showing the surface configurationsof a CBN grinding wheel observed, respectively, after grinding, truing,dressing and re-truing.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings, wherein like reference numerals designatelike or corresponding parts throughout the several views, and moreparticularly to FIGS. 1 and 2 thereof, a grinding wheel is indicated at10, which is of a configuration that an abrasive grain made of hardmaterial, such as, for example, cubic boron nitride (CBN), is bondedupon the circumferential surface of a base ring 11 with metal, resin orother bond material. The grinding wheel 10 is rotatably carried upon awheel head 14, which is slidably mounted on a bed 13. An infeed device,generally indicated at 50, is provided to effect the slide movement ofthe wheel head 14 and comprises a feed screw shaft 51, a nut 52 and aservomotor 53. A wheel drive motor, not shown, is provided upon thewheel head 14 and drivingly connected to the grinding wheel 10 so as torotate the same in a direction indicated by the arrow in FIG. 2. A table15 is slidably mounted upon the bed 13 for movement in a directionparallel with the grinding surface of the wheel 10, namely in adirection transverse to the slide movement of the wheel head 14, and isarranged to be moved by a table feed cylinder 16. A headstock 55 and afootstock 56 are disposed upon the table 15 for support of a workpiece Wto be ground with the grinding wheel 10.

Mounted on one end of the table 15 is a base 17, on which a supportcarriage 18 is slidably mounted for movement in a direction transverseto the slide movement of the table 15. The slide position of thiscarriage 18 is adjustable by manipulating a hand-wheel 25, from which ascrew shaft 26 extends for a threaded engagement with the carriage 18. Asupport spindle 20 is rotatably carried on the carriage 18 in a parallelrelation with the rotational axis of the grinding wheel 10, and a truingroll 21 for truing the grinding wheel 10 and a counter roll 22 forbacking up free abrasive grain, referred to later, in dressing of thewheel 10, are keyed on the support spindle 20 with a predeterminedseparation. The truing roll 21 is composed of a base ring, to thecircumferential surface of which diamond particles are sintered with acopper base alloy, and the width of the roll 21 is sized to be narrowerthan that of the grinding wheel 10 so as to reduce the resistance intruing. The counter roll 22 is made of refined steel, hardened steel orother material which has such hardness, toughness and resistance to wearas does a refined or hardened steel, and is designed to have a littlegreater width than the grinding wheel 10. The radius of the counter roll22, compared with that of the truing roll 21, is small within a meanssize of the aforementioned free abrasive grain for dressing, so that,when the table 15 is traversed to align the counter roll 22 with thegrinding wheel 10 after truing, a gap of a predetermined distance (t) isestablished therebetween. For example, in the case where there isemployed a free abrasive grain with a mean size of 100μ, preferably thedifference in radius between the rolls 21 and 22 is determined to be50μ, and the gap (t) becomes 50μ.

The support spindle 20 is connected via a belt driving mechanism 24 to aroll drive motor 23 mounted on the support carriage 18, and the drivemotor 23 is operable to rotate the rolls 21, 22 in such a direction asto reduce the relative circumferential speed to the grinding wheel 10. Acoolant nozzle 27 is provided on the support carriage 18, opposing thedressing roll 22, and is in fluid communication, via a pump P, with areservoir 28 containing dressing coolant. The coolant is a mixture of aconventional coolant suitable for grinding and free abrasive grain 29made of, for example, aluminum oxide, silicon carbide or other abrasivematerial. A mixing fan 31 drivable by a motor 30 is arranged within thereservoir 28 so as to maintain a proper mixing rate of the free abrasivegrain 29 with the coolant.

Fixedly mounted upon the wheel head 14 behind the grinding wheel 10 is asupport base 33, on which a slide member 34 is guided for movement in adirection parallel with the grinding surface of the wheel 10. The slidemember 34 is connected to a piston rod 36, which is extensively receivedwithin a traverse cylinder 35 formed in the support base 33, so as to bereciprocated by a predetermined stroke. On the slide member 34, there ismounted an infeed ram 38, which is intermittently advanced each time anintermittent infeed device 37, as well-known, incorporating a ratchetmechanism, is actuated. A truing bar 39, which has diamond particlessintered therein with a copper base alloy, is secured to the front endof the infeed ram 38 for effecting a re-truing on the wheel 10. Thisre-truing aims at making somewhat dull the cutting edges of the wheelabrasive grain on the wheel surface after dressing. The feed amount ofthe ram 38 by the infeed device 37 is such a distance as to infeed thetruing bar 39 by one or several microns (μ) against the wheel 10 afterdressing.

The operation of the embodiment as constructed above will hereafter bedescribed with reference to the hydraulic and electric control circuitsillustrated in FIGS. 3 and 4.

Upon completion of a previous grinding operation, the servomotor 53 ofthe infeed device 50 is operated so as to advance the wheel head 14,which is thereafter positioned at such a truing ready position that thegrinding wheel 10 is infed by a predetermined truing amount, forexample, 10μ, against the truing roll 21 if they are aligned with eachother. When a push button switch PB is depressed in this state, a relayCR1 is energized, to be self held through the closing of its normallyopen contact cr1. The relay CR1, when energized, effects theenergization of a solenoid SOL1 to switch a changeover valve 41 to aright position, and fluid under pressure is supplied into the rightchamber of the table feed cylinder 16, whereby the table 15 is movedtoward the left, as viewed in FIG. 1.

When the table 15 is moved to such a position that is immediately beforethe truing roll 21 comes into contact with the grinding wheel 10, alimit switch LS1 is actuated by means of an elongated dog 58 on thetable 15, and a contact Ls1 of the switch LS1 is closed to energize arelay CR2, which thus closes its contact cr2. Consequently, a relay CR3is energized through contacts cr1 and cr2 and is self held with itscontact cr3 now closed. The energization of the relay CR3 effects theenergization of a solenoid SOL3, and a change-over valve 42 is switched,so that the feed rate of the table 15 is reduced to a truing feed rate,suitable for truing, which is regulated by a throttle valve FV. Theenergization of the relay CR3 further effects rotation of the motor 23,and the truing and counter rolls 21, 22 are thus rotated in such adirection as to reduce the relative circumferential speed thereof to thegrinding wheel 10. In this condition, the grinding wheel 10 is truedwith the truing roll 21.

After the table 15 is traversed at the truing feed rate by an amountsufficient enough to effect truing over the entire width of the wheel10, the limit switch LS1 is switched from "ON" to "OFF," since it isreleased from the abutting engagement with the first or elongated dog58. As a result, the relay CR2 is deenergized, which allows a relay CR4to be energized through a contact cr3 of the relay CR3, a normallyclosed contact cr2x of the relay CR2 and a normally closed contact cr6x.The relay CR4, when energized, is operative to energize a solenoid SOL2,as well as to deenergize the relay CR1 through opening its normallyclosed contact cr4x. Because of the deenergization of the solenoid SOL1and the energization of the solenoid SOL2, the change-over valve 41 isswitched to the left position and, in consequence, fluid under pressureis supplied into the left chamber of the table feed cylinder 16. Thiscauses the table 15 to traverse toward the right, as viewed in FIG. 1,whereby the portions of the grinding wheel 10 which were left from beingtrued during the last leftward movement of the roll 21, are trued withthe roll 21 in this stage.

In this manner, the truing of the wheel 10 is performed through onereciprocation of traversing movement of the table 15. When the table 15,in its rightward traverse movement, is moved to such a position as toalign the counter roll 22 with the wheel 10, as shown in FIG. 1, asecond dog 59, secured to the table 15, actuates a limit switch LS2,whose contact ls2 is therefore closed to energize a relay CR5.Consequently, a relay CR6 is energized through a contact cr5 of therelay CR5 and a contact cr4 of the relay CR4, which is then deenergizedbecause of the opening of a normally closed contact cr6x of the relayCR6. This results in deenergizing the solenoid SOL2 to switch thechange-over valve 41 to the neutral position, whereby the table 15 ispositioned at a position opposing the counter roll 22 to the wheel 10.In this stage, a gap of predetermined distance (t) is establishedbetween the counter roll 22 and the wheel 10.

The energization of the relay CR6 further results in closing a contactcr6 for energization of a relay CR7, by which the pump P is operated tosupply dressing coolant from the nozzle 27 toward the gap (t). Beingbacked up with the counter roll 22, the free abrasive grains in thedressing coolant supplied to the gap (t) bite and cut away the bondmaterial of the wheel 10, whereby the same is dressed to protrude thewheel abrasive grains beyond the remaining bond material. Such dressingis continued until a time relay TR1 which has been energized togetherwith the relay CR7 is timed up. When normally closed contacts tr1x areopened with this time-up operation of the relay TR1, the relays CR7 andCR3 are deenergized to thereby discontinue operation of the pump P andthe roll drive motor 23.

With the time-up operation of the relay TR1, a contact tr1 is closed forenergization of a relay CR8. The energization of the relay CR8 effectsan operation of the intermittent infeed device 37 to infeed the truingbar 39 against the grinding wheel 10. Specifically, since in theprevious truing, the wheel 10 was infed by 10μ against the truing roll21, the infeed ram 38 is advanced by, for example, 12μ, so that thetruing bar 39 is infed by one or serveral microns against the wheel 10.

When a time relay TR2, which has been energized together with a relayCR8, is timedup, a contact tr2 is closed and a relay CR11 is energizedthrough the contact tr2 and a contact cr9, since a relay CR9 has beenenergized through a contact ls3 of a limit switch LS3, which is providedfor confirming the retracted end of the slide member 34. A solenoid SOL4is thus energized to switch a change-over valve 43 to the rightposition, and fluid under pressure is supplied into the right chamber ofthe traverse cylinder 35. The slide member 34 is traversed toward theleft, as viewed in FIG. 1, and the wheel 10 is re-trued with the truingbar 39. In this re-truing, since the infeed amount of the truing bar 39against the wheel 10 is extremely small, truing is effected on cuttingedges of the wheel abrasive grain which have been damaged by the freeabrasive grain in the previous dressing. Namely, the re-truing serves todecelerate initial wear of the cutting edges in grinding and also tomake the cutting edges somewhat dull. It is therefore to be understoodthat not only is the life of wheel 10 extended, but surface roughness ofworkpieces ground by the re-trued wheel 10 is also improved.

When the slide member 34 is moved to its left traverse end, a limitswitch LS4 for confirmation of such a traverse end is actuated, whosecontact ls4 is thus closed to energize a relay CR10, and a relay CR12 isin turn energized through a contact cr10 of the relay CR10. Since therelay CR12, when energized, causes the deenergization of the relay CR11by opening its normally closed contact cr12x, the solenoid SOL4 isdeenergized to allow the change-over valve 43 to be restored to itsoriginal position. Accordingly, fluid under pressure is supplied intothe left chamber of the traverse cylinder 35 to move the slide member 34to the original position thereof, so that the re-truing is completed.The limit switch LS3 is actuated when the slide member 34 reaches theoriginal position. A contact ls3 of the switch LS3, when closed, resultsin energizing a relay CR13, and the relay CR4 is again energized becauseof the closing of a contact cr13. With the energization of the relayCR4, the table 15 is moved toward the right in a similar manner to thatdescribed above, so as to be ready for the next grinding cycle, and whenthe table 15 reaches its grinding ready position, normally closedcontacts crAx are opened, whereby the relays CR4, CR13 are deenergized.

Although the truing bar 39, for re-truing the wheel 10 after the truingand dressing, is provided behind the wheel 10 in the foregoingembodiment, it is to be noted that the truing bar 39 can otherwise beprovided on the table 15 in a side-by-side relation with the truing anddressing rolls 21, 22. It is further to be noted that the truing bar 39can be replaced by another truing roll of the same type as the truingroll 21.

As described in detail, the present invention provides a grindingmachine with a truing and dressing device, in which the CBN grindingwheel is first trued with the truing roll, is then dressed by the freeabrasive grain backed up with the counter roll and is further re-truedwith a re-truing tool. Accordingly, the cutting edges of the wheelabrasive grain damaged by the free abrasive grain in the previousdressing are trued to be made somewhat dull, so that surface roughnessof the wheel surface can be improved, as noted by the solid line (B) ofFIG. 5. Furthermore, since the re-truing effects to decrease initialwear of the wheel surface in subsequent grinding operation, the life,namely the truing and dressing interval, can also be extended.

In FIGS. 6A to 6E, there are illustrated surface configurations of thegrinding wheel 10 observed, respectively, after grinding, truing,dressing and re-truing. The execution of a grinding operation provides aswell Sw on the wheel surface, as viewed in FIG. 6A. When the wheel 10is trued with the truing wheel 21, the swell Sw is eliminated and a newstraight profile, whose portion A is shown in FIG. 6C, appears on thewheel surface, as viewed in FIG. 6B. It is, of course, to be understoodthat the truing infeed of the wheel 10 against the truing roll 21 mustbe larger than the height of the swell Sw. With the subsequent dressing,pores on the wheel surface are deepened from a depth PD1 to anotherdepth PD2, as viewed in FIG. 6D. With the dressing, the cutting edgs ofthe weel abrasive grain 100 are excessively sharpened and damaged by thefree abrasive grain supplied between the wheel 10 and the counter roll22, and some of the wheel abrasive grain, as indicated at a referencenumeral 101, is lifted by a height Hg from the remaining bond material105. When the re-truing is performed, the cutting edges of the wheelabrasive grain 100, 101 are trued to be made somewhat dull and thecutting edge of the grain 101 so lifted is placed at the same level asthe cutting edges of other grains 100, 100 on the wheel surface, asviewed in FIG. 6E. It is therefore realized that one or serveral microns(μ) are enough for the re-truing infeed of the truing bar 39 against thewheel 10.

Obviously many modifications and variations of the present invention arepossible in light of the above teachings. It is to be understoodtherefore that within the scope of the appended claims this inventionmay be practiced otherwise than as specifically described herein.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:
 1. A grinding machine comprising in combination;abed; a wheel head slidably mounted upon said bed and adapted torotatably carry a grinding wheel made of cubic boron nitride; worksupport means for rotatably supporting a workpiece; feed means forfeeding said wheel head toward and away from said work support means;truing means including a truing roll for truing said grinding wheelthrough contact therebetween so as to reform a grinding surface of saidgrinding wheel to a predetermined profile, said truing roll beingprovided at a circumferential surface thereof with abrasive grain ashard as diamond and being rotatable about an axis parallel with therotational axis of said grinding wheel; dressing means including acounter roll and means for supplying free abrasive grain and adapted toestablish a gap of a predetermined distance between said counter rolland said grinding wheel for removing bond material from said grindingsurface in cooperation with said free abrasive grain supplied into saidgap so as to thereby protrude wheel abrasive grain on said grindingsurface from remaining bond material, said counter roll being made ofmetal and being rotatable about an axis parallel with the rotationalaxis of said grinding wheel; re-truing means including a re-truing toolfor re-truing said grinding wheel through contact therebetween so as totrue cutting edges of said wheel abrasive grain damaged by said freeabrasive grain in dressing; and control means for successively operatingfirst said truing means, then said dressing means and finally, saidre-truing means upon actuation.
 2. A grinding machine as claimed inclaim 1, further comprising:a table slidably mounted o said bed formovement in a direction transverse to the slide movement of said wheelhead and mounting thereon said work support means, said truing means andsaid dressing means, and table feed means for moving said table in thatdirection so as to selectively align said work support means, saidtruing means and said dressing means with said grinding wheel.
 3. Agrinding machine as claimed in claim 2, wherein said re-truing means ismounted on said wheel head.
 4. A grinding machine as claimed in claim 3,wherein:said means for supplying said free abrasive grain is adapted tocontinue supplying said free abrasive grain into said gap between saidcounter roll and said grinding wheel during a dressing operation; themean size of said free abrasive grain is larger than said gap; and saidtruing and dressing means are incorporated with each other and furtherinclude: a support carriage mounted on said table and carrying saidtruing and counter rolls in position to establish said gap between saidcounter roll and said grinding wheel when said wheel head is fed to sucha position as to contact said truing roll with said grinding wheel, anddrive means for rotating said truing and counter rolls.
 5. A grindingmachine as claimed in claim 4, wherein said truing and counter rollscarried on said support carriage are rotatable about a common rotationalaxis parallel with the rotational axis of said grinding wheel, andwherein the radius of said counter roll is sized to be smaller than thatof said truing roll, whereby said gap of said predetermined distance isestablished between said counter roll and said grinding wheel every timesaid truing operation is performed.
 6. A grinding machine as claimed inclaim 5, wherein said drive means is arranged to rotate said truing andcounter rolls in such a direction as to reduce the relativecircumferential speed of the same to said grinding wheel.
 7. A grindingmachine as claimed in claim 6, wherein said re-truing means comprises;asupport base fixedly mounted on said wheel head; a slide member slidablymounted on said support base for movement in a direction parallel withthe grinding surface of said grinding wheel; feed means for feeding saidslide member in that direction; an infeed ram slidably mounted on saidslide member for infeed movement transverse to the slide movement ofsaid slide member; a re-truing tool mounted on said infeed ram forre-truing said grinding wheel; and intermittent infeed means forintermittently infeeding said infeed ram so as to give said re-truingtool a predetermined re-truing infeed against said grinding wheel.
 8. Agrinding machine as claimed in claim 7, wherein said predeterminedre-truing infeed of said re-truing tool against said grinding wheel isless than half of the predetermined truing infeed by which said grindingwheel is infed against said truing roll whenever a truing operation isperformed.
 9. A grinding machine as claimed in claim 7, wherein saidre-truing tool is a diamond-impregnated truing bar.
 10. A grindingmachine as claimed in claim 9, wherein said means for supplying saidfree abrasive grain is adapted to supply toward said gap a mixture ofsaid free abrasive grain and a coolant fluid.